Patent document 1 discloses a technology in which a reference position of a control shaft is learned in a variable compression ratio internal combustion engine equipped with a variable compression ratio mechanism capable of changing an engine compression ratio in accordance with a rotational position of the control shaft. Concretely, the reference position is learned based on an output signal from a compression ratio sensor in a state where a movable part, which operates in conjunction with the control shaft, has been kept in abutted-engagement, with a stopper provided on a crankshaft bearing part that rotatably supports a crankshaft.
Patent document 2 discloses the detection of a reference position of a control shaft angle in a variable compression ratio internal combustion engine equipped with a variable compression ratio mechanism capable of changing an engine compression ratio in accordance with a rotational position of a first control shaft, while a portion of a second control shaft has been kept in abutted-engagement with a stopper provided on a housing.